Work member for a percussive tool



July 10, 1962 G. A. COOLEY 043,288

WORK MEMBER FOR A PERCUSSIVE TOOL 3 Sheets-Sheet 1 Filed June 15, 1960 .I- lill will In:

G. A. COOLEY WORK MEMBER FOR A PERCUSSIVE TOOL July 10, 1962 3 Sheets-Sheet 2 Filed June 15, 1960 y 1962 G. A. COOLEY 3,043,288

WORK MEMBER FOR A PERCUSSIVE TOOL Filed June 15, 1960 3 Sheets-Sheet 3 United States Patent 3,043,288 WBRK MEMBER 'FGR A PERUSIVE TOOL Gordon A. Cooley, Chicago, 111., assignor to Mechanical Research Corporation, Chicago, 111., a corporation of Pennsylvania Filed June 15, 1960, Ser. No. 36,302 13 Claims. (Cl. 125-36) This invention relates to a work member or spike for a percussive tool, and more especially to a self-extracting spike particularly suited for use as the concrete-penetrating work member of a paving breaker.

The conventional paving breaker structure includes a casing providing a cylinder having a piston reciprocable therein, and such piston serves as a hammer element which in the course of its re'ciprocatory movement transmits high-valued impact forces to a spike or work member supported by the casing for limited axial movements relative thereto. The spike extends outwardly from the tool casing and is provided with a pointed end that is placed in abutment with *a concrete slab or other material to be broken by operation of the tool. The spike is caused to penetrate the concrete slab by the high-valued impact forces transmitted to the spike by the reciprocating piston or hammer.

A problem of considerable significance which is encountered in the use of a paving breaker in the demolition of concrete slab is that the spike element thereof frequently becomes embedded in the concrete mass penetrated thereby and remains gripped therein by a frictional force of large magnitude (normally in the range of from 5,000 to 20,000 pounds) which is exerted on the spike by the surrounding material, and as a consequence, the spike cannot be withdrawn manually or by ordinary mechanical techniques. In such event, the workman detaches the casing from the embedded spike, inserts another spike into the casing, and then cracks or breaks the material surrounding the embedded spike so as to free the same and permit extraction thereof.

It will be apparent that such extraction operations are not only an annoyance but are also time consuming and require the availability of a plurality of spikes. It should also be mentioned in particular reference to the use of paving breakers of the ordinary type having violently vibrating casings, that a frictionally embedded spike causes substantially and often costly workman fatigue inasmuch as in attempting to release and extract such spike, the initial procedure is for the workman to manually lift the violently vibrating tool upwardly (the weight thereof being in the order of one hundred pounds) so as to develop upwardly-active vibratory impact forces between the vibrating casing thereof and the spike element in an effort to drive the spike element upwardly and out of its embedded condition, which effort as has already been indicated is by no means consistently successful.

The problem of spike extraction has been considered in Leavell Patent No. 2,752,889, which discloses an arrangement for extracting spikes which selectively utilizes the upwardly-directed movements of the vibratory tool casing to accomplish the desired spike extraction without the necessity of any manual lifting action by the worker. The present invention diflfers from the principles set forth in such prior Leavell patent in that the extraction operation is accomplished through structure comprised entirely by the spike element and is independent of the movements 3,943,283 Patented July 10, 1962 tailing structural complications therein.

Another object of the invention is in the provision of a self-extracting spike for paving breakers or the like which is characterized by developing between the embedded spike and material frictionally gripping the same a force acting upwardly on the spike and having sufiicient magnitude to consistently withdraw the same from the frictional grip of the penetrated material.

Still another object is that of providing a spike equipped with elastic energy-storage structure operative to store energy'imparted thereto by the downwardly-directed impacts received by the spike from the reciprocating hammer and then operative to apply the stored energy in reverse direction to the spike to Withdraw the same from the flictional grip of the concrete or other penetrated material.

Yet another object is to provide a conical or generally coniform or otherwise contoured spike having the characteristic of tapering toward the apex or point thereof with sufiicient acuteness so that a very short axial withdrawing movement of only a small fraction of an inch will suffice to entirely free the same from the frictional grip of the penetrated material; and which tapered spike is equipped with elastic energy-storage structure operative to store energy imparted thereto by the downwardly-directed impacts received by the spike from the reciprocating hammer and then operative to apply such energy between the penetrated material and the spike as a reversely directed force acting upwardly on the spike to move it upwardly through such short distance adequate to release it from the grip of the penetrated material.

Yet another object of this invention is in the provision of such a tapered spike having an elastic energy-storage structure for accomplishing the purpose described which includes an elastic ring circumjacent the spike and providing an interior surface engageable with the taperedsurface of the spike surrounded thereby, such circumjacent ring being adapted to be brought by the downward movement of the tapered spike into a state of abutment as between the lower surface of the ring and the upper surface of a concrete slab or other Work material being penetrated by the spike, which upper surface of such work material then resists further downward movement of the circumjacent ring so that the continued impact-impelled downward movement of the tapered spike results in strongly-forced downward movement thereof relative to and producing an energy-storing radial and consequent circumferential stretching of the circumjacent ring; the taper of the inclined area of contact between the spike and circumjacent ring being greater than the minimum self-releasing taper for the materials respectively comprising the spike and ring, wherefore such strongly-forced energy-storing radial and consequent circumferential stretching of the circumjacent ring then reactively results in an energy-yielding circumferential and consequent radial contraction thereof strongly forcing an upward movement of the tapered spike through the short distance adew quate to release it from its embedded condition in the work material.

Still a further object is that of providing a tapered spike and elastic energy-storage ring as heretofore described, which ring is resiliently related to the spike. for limited axial movement with respect thereto, such resilience being effective to protect the ring and means attaching the same to the spike from damage that could otherwise result from axial impact forces arising from the sudden arrestment of the downwardly moving spike by the concrete or other work material penetrated thereby.

Yet a further object is to provide a unique arrangement for positively and strongly attaching the energy-storage ring to the spike, which arrangement effects the aforementioned axially resilient relationship of the spike and ring; Additional objects and advantages will become apparent'as the specification develops.

An embodiment of the invention is illustrated in the accompanying drawings, in which-- FIGURE 1 is a broken side view in elevation of a spike and energy-storage ring combination embodying the invention, and in which such energy-storage ring structure respectively illustrate successive stagesin a cycle of penetration-and -extraction operation.

Description of Structure The structural exemplification of the invention illustrated in the drawings includes a spike element or work member of tapered configuration which at its outer or lower end terminates in a sharpened point 11, and at its upperendis equipped with an axially extending shank 12.

The shank 12 is generally cylindrical and has a diameter thatis substantially less than the enlarged upper end portion of the tapered spike, and an annular surface or shoulder 13 is defined by such enlarged upper end portion along the plane of its mergence with the shank 12. The surface of the spike 10 is generally conical and, as is most evident in FIGURE 1, the taper is not linear from the annular shoulder 13 to the point.11 but instead issomewhat arcuate and slightly concave outwardly, with the angle of inclination of the surface with respect to the vertical axis being less adjacent the point 11. While this concave configuration is not essential to, the present invention,

it is useful in facilitating initial penetration of the spike into a concrete slab or other work material.

The spike element 10, and particularly the shank 12 thereof, will extend into the casing of a' paving breaker (not showniand at its upper end, is adapted to receive a repetitive impacts transmitted thereto by a piston or hammer elementrin its reciprocatory movement. A heavy,

approximately conical spike of the type shown is particularly suitedfor use with a tool capable of developing and transmitting very heavy blows thereto, such as the percussive tool disclosed in the aforementioned Leavell Patent No. 2,679,826.

Cireumjacent the upper enlarged end portion of the spike 10 and lower end portion of the shank 12 is an elastic energy-storage structure 14 in the form of an annular collar or ring comprised of a materiaL'such as a properly tempered steel, having the characteristics of high strength and a degree of elasticity. In the preferred form shown,

theenergy-storage ring 14*is uninterrupted circumferentially about its entire 360 arc and has a central passage 15 extending axiallytherethrough. The upper end portion 16 of this passage is substantially cylindrical while the lower portion 17 thereof is generally frustoconical and tapers inwardly and downwardly with substantially the same taper as that of the enlarged upper end portion of i the spike 10. As is clear from FIGURE 1,'the ring 14 surrounds such upper end portion of the spike so that thesecorrespondingly tapered surfaces are oriented in adjacency. I

The included angle defined by these adjacent frustoconical areas should be equal to or greater than that defined by the minimum self-releasing taper for the unlubricated sliding abutment of the materials respectively comprising the spike and ring components, which taper in the case of steel is 1635. In the specific structural embodiment under consideration, the included angle shown as approximately is more than adequate to establish a positive self-releasing relationship between the spike and ring components, both being made of steel, with or without the applicationof a'lubricant to the adjacent surfaces thereof. 7

Intermediate the upper and lower ends thereof, the axial passage 15 has a section of enlarged diameter forming an annular channel 18 connecting with the tapered lower passage portion 17 at a horizontally disposed annular support surface 19 and with the upper cylindrical passage portion 16 through an upwardly and inwardly inclined annular preloading sin-face 20. As will be brought out in greater detail hereinafter, the angular inclination of the surface 20 provides a preloading function, and the angle formed by this surface with a horizontal plane is termed the prelo-ading angle and in this specific structure is approximately 7. 7

Communicating with the annular channel 18 through the inclined. annular surface 20, and extending downwardly and c-lockwisely thereto from the upper surface of the energy-storage ring 14, is a groove 21 of generally helical configurationcut into the cylindrical wall defining the passage portion 16 to a radial depth approximately equal to that of the annular channel ls, as will be understood from inspection of FIGURES 1, 2, and 3. The. arcuate length of the helical groove 21 is not critical, and in the present structureis approximately 75.

Mounted in the channel 18, is a stack of annular wafers or disks 22, each of which is severed or slit along a radial line 23. as is shown most clearly in FIGURES 4 and 3, and such annular stack is denoted 22S and is seen most clearly in FIGURE 1. Each of the disks 22 has a plurality of small angularly-spaced lubrication opening or apertures 24 located adjacent and distributed entirely around the outer edge portion thereof (there being three apertures separated by angles of l20- in the illustrated structure).

Each disk is angularly locked relative to the others in the stack thereof, so that (a) the apertures 24 of the respective disks are aligned and produce a plurality of angularly-spaced lubrication "bores 24S extending vertically entirely through and distributed entirely around such stack, as will be understood from FIGURESI and 3, and so that (b) the radial slits23 are distributed in equally angularly-spaced relation entirely around such annular stack. The consistency of these conditions (a) and (b) is prearranged during fabrication of such disks 22 by appropriately locating the radial slit 23 relative to F the angularly-spaced apertures 24 in each of the individual disks.

These disks, considered'separately, are thin resilient elements which may be formed of spring steel, and the relatively thick stack thereof shown in FIGURE '1 might,

for example, have a thickness of one third of an inch,

being comprised of about thirty such disks each having a thickness of approximately 0.010 inch. It will be seen that stack of disks is peripherally seated upon the annular support surface 191 whichforms the bottom wall ofthe annular channel 18, and the stack is held down-' arr-tease Wardly upon such support surface 19 by a standard radially-expansive split lock ring 25, as is made evident in FIGURE 3. The inherent resilience of the lock ring 25 causes it after installation to expand outwardly and beneath theinclined annular preload surface 20, thereby camming itself downwardly and forcing the disks 22 into tight peripheral engagement one with another and with the support surface 19. Thus, a vertically active preload is enforced on the stack of disks through the cooperative relationship of the inclined annular surface and the radially-expansive lock ring Each of the annular disks 22 defines a centrally located opening 26 of somewhat greater diameter than the spike shank 12 which, as seen in FIGURES 1 and 3, is coaxially disposed therein so as to afford an annular clearance between the disks and the shank. Each of the disks 22 is angularly locked relative to the other disks in the stack thereof by a pin 27 which prevents rotation of such stack about the spike shank 12 within the annular channel 18 defined in the energy-storage ring 14. The pin 27 is inserted in a vertical bore 27a provided in the upper peripheral portion of the ring 14, and lockingly extends through the apertures 24a provided in the respective disks to receive such pin, as is best seen in FIGURE 1. The pin 27 may be somewhat flexible and slightly bent to fixedly retain the same in its inserted position, and is preferably hollow or provided with a thread-equipped opening in the upper end thereof to facilit ate its removal for disassembly of the spike-and-ring structure. Each of the disks 22 also comprises an indexing aperture 24b to facilitate assembly of the structure as hereinafter set forth. 7

The angularly-locking apertures 24a and the indexing apertures 241) are so located during fabrication of the individual disks that vertical alignment of the indexing apertures 24b after the disks have been aggregated to form the stack 228 will at the same time establish the aforementioned arrangement of the slits 23 and vertically align the angularly-locking apertures 24a to permit reception of such pin 27. The vertical alignments of the apertures 24a and of the apertures 2417 will be regarded as respectively comprising an angularly-locking bore 24aS extending entirely through the stack of disks, and an indexing bore -4178 likewise extending entirely therethrough.

Interposed between the lowermost disk in the assembled stack 22S and the upwardly-facing annular surface 13 is a resilient doughnut-shaped element 28 that may be formed of polyurethane, or other suitably selected rub ber-like material. The doughnut 28 augments the axial resiliency between the spike 10 and the energy-storage ring 14 as provided by the stack 225 of thin flexible disks which is employed as a positive holding means for properly and permanently securing the ring to the spike; but the disk system may be designed to entirely provide the required resiliency, in which case the doughnut 28 may be replaced by a likewise separable but rigid element, or by a rigid element formed as an integral part of the spike 10 or its shank 12.

Assembly The peripheral helical groove 21 in the energy-storage ring 14 and the radial slit 23 in each of the flexible disks 22 are cooperatively employed in assembling the spikeand-ring structure. More specifically, considering the ring 14 in the unassembled condition thereof illustrated in FIGURE 2, the spike 10 is moved downwardly through a the central axial passage 15 provided therethrough until Following this, each one of the plurality of annular and normally planar flexible disks 22 is installed by first inserting the shank 12 through the central opening 26'of such disk and then temporarily deforming the disk by axially separating the two radial edges thereof as shown in FIGURE 4, so as to permit the peripheral portion of the lowermost edge, now considering FIGURES 4 and 3, to be rotated clockwisely into the open upper end 21a of the clockwisely descending helical groove 21 defined in the ring 14; and then the disk is clockwisely rotated relative to the ring 14 by an additional amount equal to the arcuate length of the groove 21 or about whereupon the aforesaid peripheral portion of such lowermost radial edge of the temporarily deformed disk will begin to emerge from the open lower end 21b of the helical groove 21 and enter the annular channel 18, so that a further additional clockwise rotation of the disk relative to the ring 14 through an arc of 360 will thread or screw the entire helical peripheral portion of the disk downwardly and into the annular channel 18. As soon as the entire 369 of its periphery has thus been delivered within the annular channel, the disk will reassume its normal planar form and fall into the place it is to occupy in the stack 225. In this manner, all of the disks 22 to be employed in aggregating the installed stack 22S thereof are successively screwed into their respective final axially related positions in the assembly as shown in FIGURE 1.

A stylus point, nail, or other similar instrument is then inserted in the indexing aperture 24b in the uppermost disk of the thus completed stack 22S thereof, and by this means such uppermost disk is rotated about the axis of the spike element 10 and relative to the next underlying disk 22 in the stack until the stylus point engages and descends into the indexing aperture 24!) in such immediately underlying disk; and this process of circular or arcuate and descending movements with the stylus point is continued until it has successively descended into and vertically aligned all such apertures 24b, at which time (1) all of the corresponding angularly-spaced lubrication apertures 24 plurally provided in the respective disks 22 will have been vertically aligned to produce a plurality of lubrication bores 243 which extend entirely through the stack 228 of such disks 22, (2) all of the radial slits singly provided in the respective disks will have been relatively positioned in angularly-spaced relation about the entire circumference of the stack, and (3) all of the angularlylocking apertures 24:: singly provided in the respective disks will have been vertically fligned to produce the angularly-locking bore 24aS which extends entirely through the stack. Continued circular or arcuate movement of the stylus point still fully inserted in the indexing bore 241:8 will then align such singularly-locking bore 24:28 with the therewith cooperative angularlylocking bore 27a provided in the peripheral portion of energy-storage ring 14, whereupon all of the disks 22 making up the stack 22S thereof are locked in their respective final angularly related positions in the'assembly by the full insertion of the pin 27 through such bore 27a and such bore 24aS as in their state of alignment.

The assembly is finally completed by radially compressing the split lock ring 25 suificiently, after locating the same about the shank 12 of the spike element, to permit the lock ring 25 to be moved downwardly through the cylindrical portion 16 of the central axial passage provided in the energy-storage ring 14. The compressed lock ring 25 is then released so that it springs outwardly beneath the inclined preload surface 20 to forcibly press the sack of disks together and downwardly upon the annular support surface 19 of the ring 14 in the firmly preloaded relationship hereinbefore described. It will be noted in FIGURE 3 that the split lock ring 25 has inwardly turned end portions 29 and 31) respectively provided with openings 31 and 32 therein for cooperation with a tool of standard character that may be used in radially compressing the lock ring. The structure can be disassembled by reversing the steps set fo h, but it is believed that disassembly will be infrequent if it is ever required, especially because the spike can be sharpened when necessarywith out removing the ring 14 and associated structure;

- Penetration and Extraction Cycle the frustoconical bottom surface 35 of the energy-storage ring 14 is brought into initial contact with the upper surface of the slab, as shown in FIGURE b. In most cases the high-valued lateral forces developed within the slab by the wedging action of the downwardlymoving tapered spike will cause the slab to fracture along a generally vertical inter-face, and in FIGURES 515 through 5f such fracture is indicated by the line 36. It should be mentioned, however, that the fractural separation producedby this powerful wedging action often becomes substantially wider than a merely detectable crack, and in such cases removal of the concrete fragment which has been separated from the; 'mainmass of the slab is materially facilitated.

As operation of the tool is further continued, the highvalued downwardly-acting forces developed by the im 3 I thereby produced progressiye' radial and consequent circumferential stretching of theriug, necessarily in association with a progressive, elastic storage therein of potential mechanical energy which, because of the self-releasing taper of the contiguous surfaces of the spike and ring andbecause ofthe reactive operation of the'pro gressively stretching and progressively resisting ring, ap-

plies to the spiketa continuous and progressively increasing upward axial force, which, when it has developed to equality with or to slightly exceed the less rapidly increasing frictionally established axialforce' theretofore retaining thev tapered spike in its embedded condition in the concrete, triggers the discharge of such elastically-stored potential mechanical energy, whereupon, through the agency of the circumferential and consequent radial contraction of the ring, this reversely-acting energy'forcibly impels the taperedtspike 10 through the short upward axial movement adequate to free it from the frictional grip of the concrete, as shown in FIGURE 5e.

Since the quantity of such discharging mechanical energy which thus impels the spike through the short upward axial movement adequate to free it from its embedded condition in the concrete mass ordinarily substantially expacts delivered to the spike will impel its further descent into the concrete slab 34, so that the ring 14 carried downwardly therewith will forcibly and progressively impress its frustoconicalbottom surface into the upper surface 33-of the slab, forming therein a generally matingt seat 37 for such bottom surface 35 of the ring, as shown in FIGURES 50 through 5 The purpose of the frustoconical contour of this bottom surface 35 of the ring is'to minimize the lateral or radial thrust which would otherwise develop to undesirably high valuesbetween portions of'the upper end of the spike element 10 or of its shank 12 and therewith slidably cooperative surfaces internally comprised by the tool, in case such bottom surface of forces applied thereagainstthrough and by the energystorage ring 14; As a'consequent' of the occurrence of this condition, downward movement of the energy-storage ring '14 terminates, but the spike element 10 continu'esto be driven deeper into the concrete by the highvalued impact forces developed in the tool, and progres-' sively moves downwardly relative to the ring '14 and into the configuration therewith which is illustrated in FIG- URE 5d. The extent of the downward movement of the spike relative to the ring is exaggerated in FIGURE 5d for purposes'of graphic illustration; and the extent of such movement is readily apparent by noting, as between the broken lines 38, indicating the lowermost plane of the frustoconical bottom surface 35 of the ring, and the annular surface 13 defined on the spike element 10.

Quite evidently this strongly-forced progressive down- 7 ward axial movement of the tapered spike ltl deeper into the concrete and relative to the closely circumjacent energy-storage ring 14 can only occur coincidently with a FIGURES 5c and 5d, the variation of the distance between ceeds the quantity of energyv actually utilized in effecting this short movement of the spike against the frictional force tending -to retain it in the concrete mass, it follows that a considerable part of suchdischarging energy becomes, invested in imparting-a not inconsiderable upward velocity to the self-extracting spike structure, sothat the ring'14 normally jumps entirely clear of the upper surface of the concrete slab by areadily appreciable or conspicuous amount, as shown in FIGURE 5 thus'signalling the operator-that the cycle of penetration-and-extraction operation has been completed, and that the spike point may be relocated for commencement of asubsequent penetra- V In the event that the-self-extracting spike is to be used with a percussive tool capable of developing only relatively light or low-valued repetitive impact forces for transmission to the spike, it may be desirable to reduce thefriction developed between the contiguous self releasing taper surfaces of the spike and ring beyond the reduction in friction which can be accomplished by lubrieating such surfaces as by filling the space 39 by introducing thereinto a drylubricant through one or moreof the bores 248; Such additional reduction in the friction between the contiguous surfaces can be provided by lining the tapered surface of the ring with a'material having a relatively low coefficient of friction as, for example, with a syntheticresin plastic such as nylon or a synthetic rubber such as polyurethane. In either event, the preferable condition is to impregnate the linear with a dry lubricant such as, for example, graphite or molybdenum disulfide, Lubricants of this same type may be used for directly lubricating. the unlined contiguous surfaces of the spike and ring. It may be noted that while from the point of view of-friction reduction either or both of the contiguous surfaces of the spikeand ring could be equipped with a friction-reducing liner, practicably it is difiicult to maintain a bondbetween thespike' and such liner therefor because of the repetitive impact forces transmitted to the spike. 7 v

While I have in this'specification, for'purposes of illustration, described in considerable detail one specific embodiment of myinvention, it will be understood that many changes in details can be made by those skilled in the art without departing from the spirit of my invention.

I claim;

1. A self-extracting work member structure for use with a percussive tool operative to transmit repetitive highvalued impact forces thereto, comprising a work member element for penetrating concrete'slabs and the like in responseto such impact forces transmitted thereto and having a tapered slab-penetrating portion that may become frictionally gripped by such penetrated slab, the taper of of said slab-penerating portion so as to be brought into abutment with such penetrated slab upon penetration thereofto a predetermined depth by said slab-penetrating portion, said energy-storage structure as a result of such abutment being operative to develop between said work member element and penetrated slab an axial force displacing said work member element through such limited axial withdrawing movement and thereby releasing the same from the frictional grip of such penetrated slab.

2. The self-extracting work member structure according to claim 1 in which the acute angle of said taper is selected to restrict said limited axial withdrawing move ment to a value not exceeding one-half of an inch.

' 3. A self-extracting work member structure for use with a percussive tool operative to transmit repetitivehighvalued impact forces thereto, comprising a Work member element for penetrating concrete slabs and the like in response to such impact forces transmitted thereto and having a tapered slab-penetrating portion that may becomefrictionally gripped by such penetrated slab, the taper of such slab-penetrating portion being acute so as to cause release thereof from any such frictional grip of the penetrated slab in conseqeunce of a limited axial withdrawing movement of said work member element from its position of deepest penetration into such slab, and an elastic energy-storage structure carried by said work member element so as to be brought into abutment with such penetrated slab upon penetration thereof to a predetermined depth by said slab-penetrating portion, said energy-storage structure being operative as a consequence of and subsequent to such abutment to develop between said work member element and such penetrated slab an axial force displacing said work member element through such limited axial withdrawing movement and thereby releasing the same from the frictional grip of such penetrated slab.

4. The self-extracting work member structure according to claim 3 in which said elastic energy-storage structure comprises an annular element circumjacent said tapered slab-penetrating portion adjacent the maximum diameter thereof, said annular element being circumferentially stretched by the continued slab-penetrating displacements of said work member element following abutment of said annular element with such penetrated slab to develop the aforesaid axial withdrawing force.

5. The self-extracting work member structure of claim 3 in which said annular element is provided with a tapered passage therethrough matingly receiving said slab-penetrating portion of said work member element therein, the mating tapered surfaces of said annular element and work member element defining a self-releasing taper therebetween.

6. The self-extracting work member structure of claim .5 in which said annular element is a continuous ring.

7. A self-extracting work member structure for use with a percussive tool operative to transmit repetitive high-valued impact forces thereto, comprising a work member element for penetrating concrete slabs and the like in response to such impact forces transmitted thereto and having a tapered slab-penetrating portion that may become frictionally gripped by such penetrated slab, the taper of said slab-penetrating portion being acute so as to cause release thereof from any such frictional grip of the penetrated slab in consequence of a limited axial withdrawing movement of said work member element from its position of deepest penetration into such slab, and an elastic energy-storage structure carried adjacent the upper end of said slab-penetrating portion so as to be brought into abutment with such penetrated slab upon penetration thereof to a predetermined depth by said slab-penetrating portion, said energy-storage structure subsequent to such abutment being engaged by said work member structure and having force imparted thereto during the continued slab-penetrating displacements of said work member element, said energy-storage structure being operative to store such force as potential mechanical energy until the value thereof exceeds the value of the frictional grip of such penetrated slab and to then develop between said work member element and penetrated slab an axial force displacing said work member element through such limited axial withdrawing movement to thereby release.

the same from the frictional grip of such penetrated slab.

8. The self-extracting work member structure according to claim 7 in which said elastic energy-storage structure comprises an annular element circumjacent said tapered slab-penetrating portion adjacent the maximum diameter thereof, said annular element being circumferentially stretched by the continued slab-penetrating displacements of said work member element following abutment of said annular element with such-penetrated slab to develop the aforesaid axial withdrawing force.

9. The self-extracting work member structure of claim 8 in which the annular element is provided with a tapered passage therethrough matingly receiving said slab-penetrating portion of said work member element therein, the mating tapered surfaces of said annular element and work member element defining a self-releasing taper therebetween.

10. The self-extracting work member structure of claim 9 in which said annular element is a continuous ring.

11. The self-extracting work member structure according to claim 7, and further comprising a resilient mounting structure securing said elastic energy-storage structure to said work member element and affording limited axial movements therebetween to prevent damage to said elastic energy-storage structure that could otherwise result from axial impact forces imparted thereto by the sudden arrestment of the slab-penetrating motions of said work member element.

12. The self-extracting work member structure of claim 11 in which said mounting structure includes a plurality of flexible disks oriented one upon another to define a stack thereof, said stack being fixedly secured to said elastic energy-storage structure and related to said work member element through the resilience of said stack.

13. A self-extracting Work member structure for use with a percussive tool adapted to transmit repetitive highvalued impact forces thereto, comprising a work member element adapted to penetrate concrete slabs and the like in response to such impact forces transmitted thereto and having a tapered slab-penetrating portion sometimes frictionally gripped by such penetrated slab, the taper of said slab-penetrating portion being sufficiently acute to cause release thereof from the frictional grip of such penetrated slab in consequence of a limited axial withdrawing move ment of said work member element from its position of deepest penetration into such slab, an elastic energy-storage structure adapted to be brought into abutment with such penetrated slab by the slab-penetrating displacements of said work member element for developing between said work member element and penetrated slab an axial force of sufficient magnitude to move said work member element through such limited axial withdrawing movement and thereby release the same from the frictional grip of such penetrated slab, and resilient mounting structure securing said elastic energ -storage structure to said work member element and affording limited axial movements therebetween to prevent damage to said elastic energystorage structure that could otherwise result from axial impact forces imparted thereto from the sudden arrestment of the slab-penetrating motions of said work member element by the slab being penetrated thereby, said mounting structure including a plurality of flexible disks oriented one upon another to define a stack thereof fixedly nular ring having aradially extendingslit theretbrough,

said elastic energy-storage structure including an annular element having a passage extending therethrough receiv-' ing said work member element therein, said passage being prqvided, intermediate the ends thereof'with an enlarged annularchannel having a diameter approximately equal to the outer diameter of each of said disksfor the circumjacent and axialienclcsureflof the peripheral portions thereof, said passage also being provided with a helical groove connecting saidv channelfand screwingly providing axial access to said channel. fOr effecting such enclosure therein of each of said-disks References Citedin. the fileofvthis patent UNITED STATES PATENTS Jowett Dec. 9, 1930' 

